Syngas production from partial oxidation of methane over Ce1−XNiXOY catalysts prepared by complexation–combustion method

Applied Catalysis A: General 311 (2006) 24–33 www.elsevier.com/locate/apcata

Syngas production from partial oxidation of methane over Ce1
XNiXOY catalysts prepared by complexation–combustion method Wenjuan Shan a,*, Matthieu Fleys a, Francois Lapicque b, Dariusz Swierczynski c, Alain Kiennemann c, Yves Simon a, Paul-Marie Marquaire a,* a

De´partement de Chimie Physique des Re´actions, CNRS-ENSIC, BP 20451 F-54001 Nancy, France b Laboratoire des Sciences du Ge´nie Chimique, CNRS-ENSIC, BP 20451 F-54001 Nancy, France c Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, CNRS-Universite´ Louis Pasteur, 25 rue Becquerel, F-67087 Strasbourg, France Received 30 March 2006; received in revised form 28 May 2006; accepted 29 May 2006 Available online 17 July 2006

Abstract Synthesis gas production from partial oxidation of methane (CH4/O2/Ar = 4/2/94) over Ce1
XNiXOY (X = 0.05–0.6) catalysts was investigated between 500 and 900 8C by using a jet stirred reactor. The highest conversion rate of CH4 and O2 was obtained on Ce0.6Ni0.4OY where the selectivity of hydrogen was close to 80% and the CO selectivity was higher than 90%, although the residence time was very low (d = 3 s). The desired ratio H2/CO about 2 was obtained at about 700 8C for all catalysts. Moreover, the catalyst exhibited good stability as no reaction activity decrease was observed during 160 h reaction at 650 8C due to the high redox property of the catalyst in the reaction. The structure and property of the catalysts were investigated by XRD, TPR, BET, N2-adsorption, and XPS for the fresh and used samples. The TPR and XPS results suggested that Ce3+/Ce4+ and Ni2+/Ni0 couples coexisted in the reaction by Ni2+ + Ce3+ + & ! Ni0 + Ce4+ due to the strong interaction between the Ni and Ce species in the catalysts. The lattice oxygen actived by Ce3+ with oxygen vacancy is the main active oxygen species for the reaction. Both Ni0 and Ce3+ with oxygen vacancy are the active sites for the reaction. The active site transferred between Ni0 and Ce3+-& depending on the availability of lattice oxygen in the reaction. # 2006 Elsevier B.V. All rights reserved. Keywords: Syngas; Partial oxidation of methane and Ce1
XNiXOY

1. Introduction In the recent decades, the conversion of nature gas to valuable chemicals was paid more attention, such as oxidation couple reaction [1–4], selective oxygen of methane to methanol or to formaldehyde [5–7] and methane dehyro-aromatization [8–10]. But up to now, the most economically available route is the conversion of methane to synthesis gas, and steam reforming of methane is the widely used route in industry [11]. However, the route is expensive because of its endothermic background, low space velocities and higher

* Corresponding authors. E-mail addresses: [email protected] (W. Shan), [email protected] (P.-M. Marquaire). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.05.044

H2/CO ratio for downstream products, which is not suitable for the synthesis of methanol or the long-chain hydrocarbons made in the Fischer–Tropsch process. Thus partial oxidation of methane has been under intense study as a potential alternative to the steam reforming process. All the VIII metals in periodic table have been reported as active catalysts for the partial oxidation of methane. Prettre et al. [12] first reported the formation of synthesis gas via the catalytic partial oxidation of CH4 catalyzed by a 10 wt% refractory supported nickel, at temperatures between 973 and 1173 K in the 1940s. However the main disadvantage of this technique was the use of pure oxygen which represented up to 40% of the cost of a syngas plant, the rapid deactivation due to the carbon deposition and metal loss at higher temperature [13]. To decrease the carbon deposition and increase the stability of the support, which can extend the catalyst lifetime, much work

W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

has been focused on the modification of support [14–17], the effective promoters [18,19] and novel preparation methods, such as a solid phase crystallization method, a sol–gel method and a citrate method [20–22]. It was found that the carbon deposition depends on the Ni dispersion and stability on supports [23]. It also was suggested that the degree of reducibility of nickel [24] as well as the basicity of the support substrate [14,25] are key requirements to achieve very high hydrogen yields. The addition of rare earth metal oxide or alkaline metal oxide to alumina or the use of rare earth metal oxide as support can restrict carbon deposition [11]. The gain brought by the presence of rare earth oxides in the catalyst support is probably the fact of its capability for oxygen storage, which can reduce the amount of carbon deposited by its effective oxidation. It is also believed that the presence of a rare earth oxide such as CeO2 can stabilize the support and prevent it from sintering during the high-temperature reaction. It was suggested that CH4 dissociates on Ni surfaces and the resulting carbon species migrates to the Ni-CeO2 interface before reacting with lattice oxygen of CeO2 to form CO. A synergistic effect between Ni and CeO2 support contributes to CH4 conversion [26]. In addition, the synergistic effect of the highly dispersed nickel-ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface, effectively oxidizing any carbon species produced from methane dissociation on nickel [17]. In this paper, we report the performance allowed by new Ce1
XNiXOY non-supported catalysts for partial oxidation of methane, using a jet stirred reactor at low residence time. The structure and property of catalysts were investigated by XRD, TPR and XPS. The interaction of active Ni species and CeO2 lattice oxygen mobility was discussed depending on the characterization results. 2. Experimental 2.1. Catalysis preparation

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out using a TOPCON-EM002B apparatus coupled with an energy dispersion X-ray device (EDS) operating at 200 kV. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D-5000 powder diffractometer using nickel-filtered Cu Ka ˚ ). The Ce, Ni and C contents of the solids radiation (1.5406 A were measured by elemental analysis performed at the laboratoire central d’analyse-CNRS in Vernaison (France). Temperature programmed reduction (TPR) was conducted using a conventional apparatus equipped with a TCD detector. A 50 mg sample placed in a U-shaped quartz tube (6.0 mm i.d.). TPR was performed by heating the samples at 15 8C/min from 25 to 900 8C in a 3% H2–N2 mixture flowing at 50 ml/min. X-ray photoelectron spectra (XPS) were acquired on a Multilab 2000 spectrometer (Thermo VH Scientific) using Al Ka radiation (1486.6 eV). The aluminum anode was operated at an accelerating voltage of 15 kV, 15 mA, 20 Votter. Base pressure in the analysis chamber was maintained in the range of 5  10
9 mbar. 2.3. Catalytic activity Partial oxidation of methane reaction was performed by using a jet stirred reactor in the temperature range from 500 to 900 8C under 800 mmHg (106.6 kPa). A low residence time is available by using the reactor. The cylindrical catalyst pellet whose weight equals to 350 mg catalyst (one pellet) was loaded into the reactor. The reaction feed containing 4 vol% methane, 2 vol% oxygen and balanced by Ar was introduced with a residence time about 3 s. The outlet gaseous mixture was analyzed using an on-line gas chromatograph (Agilent 3000 A Micro GC) equipped with TCD detectors. The conversion of methane was calculated. X CH4 ¼

CH4;in
CH4;out CH4;in

(1)

The selectivity of CO can be defined as the molar ratio of CO produced to that of CO2 and CO, if one assumes that no solid

The Ce1
XNiXOY materials were prepared by citric acid complexation–combustion method [27]. Ce(NO3)36H2O and Ni(NO3)23H2O (Aldrich) were dissolved in deionized water in such a manner to give solution of 1.0 M. Citric acid (Alfa Aesar) was added with 1.2 times molar amounts to the premixed nitrate solutions of cerium and nickel with different Ni/ Ce ratios. The solution temperature was kept at 70 8C for 2 h. Once the gel formation, the temperature was elevated to about 150 8C quickly, the gel foamed with production of nitrogen oxide vapors and combustion started with sparks. A solid product was obtained after the sparks were extinguished. The resulting powder was calcined at 700 or 800 8C for 5 h in air. The notation of X, Y in the samples means the atomic ratio. 2.2. Characterization The specific area of the samples was obtained at 77 K using a Sorptometer Coulter SA 3100. TEM investigations were carried

Fig. 1. Catalytic activity of partial oxidation of methane over Ce0.8Ni0.2OY catalysts.

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W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

carbon is formed: SCO ¼

CO CO þ CO2

3. Results (2)

Whereas the selectivity of hydrogen is defined taking into account the stoichiometry of the partial oxidation of methane: SH2 ¼

H2 2X CH4 CH4

(3)

The variables involved in the above formulae corresponds to the partial pressures of the various reactants, which are proportional to the mole flux, assuming perfect gas behavior.

3.1. The studies of catalytic activity over Ce(1
X)NiXOY prepared by complexation–combustion method Fig. 1 shows catalytic activity of partial oxidation of methane over Ce0.8Ni0.2OY-cc catalyst. The conversion of CH4 and O2 increases with the increase on temperature. The conversion of O2 is higher than that of CH4 at all temperatures detected. Below 700 8C, the two conversions exhibit similar variations with temperature, and over this limit, the conversion of oxygen increases faster than that of methane. CO2 content decreases with temperature below 700 8C, which obtains the lowest value of 0.12% at 700 8C, in agreement with the

Fig. 2. (a–e) The influence of Ni content in Ce1
XNiXOY-cc catalysts on the catalytic behavior.

W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

Boudouard equilibrium; at higher temperature, CO2 concentration increases again, corresponding to a higher significance of combustion. The content of H2 and CO increase regularly with temperature; however the H2/CO ratio decreases with temperature and the suitable syngas ratio of 2 is attained at about 700 8C. In all the tested temperatures, a carbon balance higher than 90% is available as shown in Fig. 1. The influence of Ni content in catalysts on the catalytic behavior was investigated. As shown in Fig. 2, the conversion of CH4 and O2 increased slightly with the increase of Ni content, but it is not proportionate to the increase of Ni content. The highest value was obtained at the point of Ni content with 0.4. A decreased activity was found for the case of further increase Ni content up to 0.6. For the selectivity of H2, depends on not only Ni content, but also reaction temperature. It increases with temperature, exhibits a maximum in the range 600–650 8C and decreases strongly with further increase of the temperature. At temperatures below 600 8C, the highest selectivity is obtained with the highest Ni content. While at higher temperature, the Ce0.6Ni0.4OY shows highest H2 selectivity, although the selectivity decreases with temperature in this range. For the CO selectivity, it shows a behavior differing somewhat from that of H2 selectivity, as shown in Fig. 2d, which increases with the temperature. A stable value was obtained over a temperature threshold. This threshold is reduced noticeably with higher Ni contents. The highest selectivity of CO about 90% was obtained at 650 8C in Ce0.6Ni0.4OY sample. Based on these results, it can be concluded that the Ni content showed slight affect on the conversion of methane and oxygen, but it showed strong influence on the distribution of products, especially at lower reaction temperature. At higher temperature (650 8C), the conversion of both methane and O2 increased sharply with temperature. O2 conversion rate is quite faster than that of CH4. But no complete conversion of O2 was found even at 900 8C indicating that the molecular oxygen does not react directly with CH4, instead, it was activated on the oxygen vacancy to replenish the lattice oxygen on CeO2. CO formation rate increased due to the rapid diffusion of lattice oxygen and desorption of products from the surface of catalysts at high temperature. The ratio of H2/CO decreases with temperature for all the samples, as shown in Fig. 2e. The effect of the catalyst composition was significant at low temperatures, at 500 8C, it decreases from 7.5 to 3 when the Ni content corresponding on support increase from 0.05 to 0.6. At temperature lower than 650 8C, selectivity of H2 and CO increase with temperature, and they also increase with Ni content at fixed temperature. The ratio of H2/CO decreases with temperature and Ni content. The effect of Ni content in the catalysts was little significant over 650 8C, especially for the case of temperature higher than 700 8C. The ratio of H2/CO is very close and lower than 2.0. The desired H2/ CO about 2 was obtained at about 700 8C for all samples. The experiment about the products distribution as function of stream on time was performed over Ce0.8Ni0.2OY catalyst at 500 8C. The conversion of both methane and oxygen were found to increase regularly over the 3-h run, as shown in Fig. 3.

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Fig. 3. The partial oxidation of methane over Ce0.8Ni0.2OY catalyst calcined at 800 8C for 5 h, reaction at 500 8C.

X CH4 passed from 8 to 21% whereas X O2 increased from 16 to 30%, the highest conversion of oxygen corresponded to significant production of CO2. Significant changes in the gas composition were observed with time. Production rates of hydrogen and carbon monoxide increased with time, especially in the first 100 min. H2/CO ratio exhibited a maximum shortly after starting the experiment, at 4.4, and decreased significantly in the first hour, after this lapse of time, the rate of decrease was slower and the ratio attained 3.4 after 3 h. The production rate of CO2 was more constant in the run. In the first hour of the run, CO2 concentration was significantly higher than that of CO, which was however increasing with time. After 1 h, the inverse trend was observed, with more CO produced. This phenomenon, together with the compared variations of the conversions of methane and oxygen, show that in the first part of the experiment, the total oxidation of methane – i.e. its combustion – occurred along the partial oxidation. The side oxidation became less significant with time, as indicated by the closer values of the conversions and the predominating CO. During the reaction process, the existing NiO active for total oxidation of methane, might be reduced to Ni, which is more active for both partial oxidation and steam reforming of methane. 3.2. The structural studies of the catalysts Fig. 4a shows the XRD patterns of fresh Ce1
XNiXOY-cc calcined at 700 8C in air for 5 h. The XRD pattern related to pure CeO2 is composed of sharp peaks. For Ce1
XNiXOY (X = 0.05–0.6), the peaks ascribed to CeO2 are weaker than those of pure CeO2. This indicates that the CeO2 lattice was distorted due to the presence of Ni atom in the Ce1
XNiXOY structures, or that Ni can inhibit the sintering of CeO2. It also suggests that the existence of strong interactions between Ce and Ni atoms in the catalysts prepared by complexation– combustion method. The diffraction peaks of NiO appeared in the catalysts when Ni content is higher than 0.2. As expected the intensity of the peaks increased with the Ni content.

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W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

Fig. 4. XRD patterns of Ce1
XNiXOY-cc catalysts. (a) Fresh samples calcined at 700 8C in air for 5 h, (b) after reaction (500–900 8C).

XRD patterns of the used samples are shown in Fig. 4b. All the peaks ascribed to CeO2 were observed to be sharper than those exhibited by the fresh materials, and this could be attributed to a change in crystallite size induced by the higher reaction temperatures, in comparison to that in the calcination procedure. No diffraction peaks ascribed to NiO were observed for all the used samples, whereas Ni diffraction peaks are visible for X > 0.3 (Fig. 4b). Since the NiO was reduced to Ni in the reaction atmosphere at high temperatures (>600 8C), as reported by Choudhary et al. [28]. Table 1 lists the BET surface and elemental analysis for the samples. It was found that the Ce1
XNiXOY-cc catalysts have a low BET surface about 10–20 m2/g. On the whole, the specific BET surface decreases as Ni content increases in the samples. BET surfaces of the used samples are lower than the ones corresponding fresh catalysts, In accordance with XRD results, this due to the catalysts sintered during reaction at high temperatures (from 500 to 900 8C). However, use of the catalysts did not result in significant modification of their stoichiometry, as shown in an example in Table 1.

3.3. Redox property of Ce1
XNiXOY catalysts The TPR profiles of Ce1
XNiXOY catalysts calcined at 700 8C for 5 h are shown in Fig. 5. All H2 consumption peaks were divided into two parts, i.e., low temperature peaks (<600 8C) and high-temperature peak (>600 8C). For fresh samples (Fig. 5a), the a peak appearing at 250 8C or so corresponds to the reduction of adsorbed oxygen in CeO2 vacancies as suggested by Shan et al. [29]. The radius of Ni2+ is smaller than that of Ce4+, and the two ions have different oxidation state. When Ni2+ is incorporated into the lattice of CeO2 to replace some Ce4+ cations, a solid solution is formed. The unbalance of charge and lattice distortion happen within the structure of CeO2, and this leads to the generation of oxygen vacancies, which adsorb oxygen easily. Therefore, very reactive oxygen species on the catalyst are formed, which are reduced easily by H2 at low temperature. In the solid solution, CeO2 cubic structure is not changed so much by the incorporated Ni2+. However, the Ni2+ ions incorporated in solid solution are more difficult to be reduced [30]. So, there is no hydrogen

Table 1 BET surface area and elemental analysis results of Ce1
XNiXOY samples X in Ce1
X NiXOY-cc

Fresh samples (m2/g)

Used samples after reaction at 900 8C (m2/g)

Ni wt% calculated

Ni wt% exp.

Ce wt% calculated

Ce wt% exp.

Bulk Ni/Cea

0 0.05 0.1 0.2 0.3 0.4 0.6 Used—0.1

20.9 21.1 23.8 14.4 14.3 10 10

– 5.5 5.8 5.6 5.4 3.2 2.6

– 1.76 3.64 7.73 12.39 17.72 31.11 3.64

– 1.71 3.40 8.23 12.06 17.52 30.91 3.46

– 79.57 77.63 73.39 68.58 63.06 49.21 77.63

– 78.27 75.82 71.21 66.80 63.90 49.07 77.47

0.05 0.11 0.27 0.43 0.65 1.47 0.11

a

The data were obtained from elemental analysis.

W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

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Fig. 5. TPR profiles of Ce1
XNiXOY catalysts. (a) Fresh samples, (b) used samples.

consumption peak corresponding to the reduction of Ni2+ species incorporated into solid solution in the TPR profiles. A broad peak named b peak, including an ensemble of several shouldered peaks, shifting to higher reduction temperature as the Ni content was increased, which is ascribed to the reduction of NiO in the catalysts, covering both the reduction of highly dispersed NiO and the step by step reduction of aggregated NiO on the surface of CeO2. g-Peak appearing near 800 8C, is related to the reduction of lattice oxygen atoms in CeO2. For the used samples, the low temperature reduction peaks are much weaker when compared to the fresh samples, indicating that NiO is reduced to Ni in the reaction process as shown in XRD patterns (Fig. 4b). And the reduction temperature is slightly lower than that of fresh samples. The border of a and b disappeared after the reaction. While, the g peak, ascribed to the reduction of CeO2 lattice oxygen, is stronger than that corresponding fresh samples. These phenomena indicated that the redox cycle for both Ni and Ce species happened during the reaction.

The amounts of hydrogen consumed for the TPR experiments were reported in Table 2. It was found that the amount of H2 used in the reduction at low temperatures is higher than that required for the specific reduction of NiO, estimated by calculation. This suggested that a part of CeO2 can be reduced at lower temperature due to the H2 spillover from Ni to CeO2, which is in good agreement with the literature [31–33]. For the used samples, the H2 consumption of lower temperature peaks was much lower than that of fresh samples. This is due to most of NiO was reduced during the reaction, while a part of Ni2+ still existed during the reaction even at 900 8C. The ratio of unreduced NiO to total NiO decreases with the increase of Ni content due to the strong interaction between Ce and Ni in solid solution formed in lower Ni content samples, as suggested by Wrobel et al. [30]. However the re-dispersion of this part NiO happened in the reaction, it can be reduced by H2 at lower temperature (Fig. 5b). This indicates that both NiO and partly reduced Ni are necessary in the POM reaction, and different Ni species play different roles on the product distribution.

Table 2 H2 consumption (ml) during TPR experiments Samples

CeO2 Ce0.95Ni0.05OY-700-cc Ce0.9Ni0.1OY-700-cc Ce0.8Ni0.2OY-700-cc Ce0.7Ni0.3OY-700-cc Ce0.6Ni0.4OY-700-cc Ce0.4Ni0.6OY-700-cc

Fresh samples

Used sample

Calculated value

a and b

g

a and b

g

H2 (ml) needed for NiO

Unreduced NiO/NiOtol (%)

H2 (ml) needed for CeO2

Reduction extent of CeO2 (%)

0.48 0.84 1.76 2.42 3.60 6.13

1.14 0.88 0.84 0.70 0.64 0.55 0.40

0.08 0.10 0.30 0.29 0.22 0.56

0.83 0.99 0.60 0.78 0.78 0.64

0.33 0.66 1.58 2.35 3.31 5.89

24 15 18 12 7 9

3.26 3.19 3.12 2.88 2.74 2.55 1.97

35.0 32.2 32.7 30.6 25.9 32.9 32.5

Note: For the calculated value, it is supposed that NiO can be reduced to Ni completely, and all the Ce4+ can be reduced to Ce3+ (error  5%).

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W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

Fig. 6. The reduction extent of CeO2 lattice oxygen in Ce1
XNiXOY catalysts.

The behavior of CeO2 shows large different from that of NiO as only part of CeO2 is found to be reduced. Available reduction extent of CeO2 is about 30–35% depending on the samples (listed in Table 2). The H2 consumption of CeO2 lattice oxygen decreased with the increase of Ni content for the fresh. The H2 consumption of used samples is higher than that of fresh samples, especially for the higher Ni content samples. To understand the influence of Ni content on the oxygen species, Fig. 6 shows the reduction extent of lattice oxygen as function of Ni content. It is found that the reduction extent of bulk CeO2 decreases with the increase of Ni content, whereas the inverse tendency was observed with the used materials. Nevertheless, the reduction degree of CeO2 is higher that corresponding fresh samples, ranging from 25 to 35% depending on the catalyst considered. Considering the reduction behavior before and after reaction, it was suggested that oxygen transfer happens between Ni and Ce during the reaction, i.e., Ni2+ + Ce3+ + & + e
$ Ce4+ + Ni0. Therefore, NiO in the catalyst was partially reduced to metallic nickel during reaction. The catalyst is active and selective in this partially reduced state. 3.4. The chemical states study of elements in Ce0.9Ni0.1OY catalysts XPS was employed in order to obtain information about the chemical state of elements and their relative concentration on

the surface of Ce-Ni-O catalysts after calcination and after reaction. Core level spectra including O 1s, Ni 2p and Ce 3d were recorded for calcined and used catalyst samples. C 1s was used as the internal reference. Table 3 summarizes band energy and relative atomic concentrations of the elements obtained from XPS results. The peaks ascribed to band energy of Ni species are very weak due to the low Ni content in samples. The band energy is in a broad range centered at 855.4, with a satellite peak at 861.7 eV. This value is higher than the binding energy (BE) of free NiO reported at 854.2 eV [32], which indicated the strong interaction between the Ni species and CeO2. According to the values found in the literature, the components at 852.3 and 868.0 eV correspond to the main and satellite peak of Ni0, respectively. The component at 855.4 eV (with two satellites at 861.7 and 873.6 eV) is attributed to NiOc2+ whereas the component with BE of 858.1 eV is ascribed to NiT2+ [33]. Therefore, the feature observed in XPS measurements with fresh and used catalysts were attributed to NiOc2+, this Ni species has an octahedral coordination with oxygen similarly to CeO2. The proportion of Ce4+ and Ce3+ is difficult to obtain due to the very similar energies of the 4f and orbital ligand valence level. The peak located at BE  916.7 eV is typical of Ce4+ and absent for Ce3+, its intensity is to decrease along the reduction, which has been used frequently with this aim. The percentage of the total integrated spectrum intensity lying within this peak in pure, non-reduced CeO2 has been computed to amount to 13.7% [34]. A reduced CeO2 surface is confirmed by a decreasing ratio. It was 9.3 and 10.5 for fresh and used Ce0.9Ni0.1OY-cc, respectively, as shown in Table 3, which indicated that a lot of Ce4+ ions were reduced due to the present of Ni and part of them is re-oxidized in the reaction. From O 1s spectra, two kinds of oxygen species are found, with signals at 529.7 and 532.1 eV for the fresh sample, and 530.7 and 533.0 eV for the used sample. They were ascribed to lattice oxygen and adsorbed OH
1 or CO3
2 as suggested in literature [35–37]. In this case, the band energy is slightly higher than that reported in the references, which is due to the present of Ce3+ and the strongly interaction between Ce and Ni species. Moreover, the surface O/Ce ratio in used sample is higher than fresh sample, indicating that part of Ce3+ in fresh sample can be oxidized to Ce4+ during the reaction, which is a good agreement with TPR results.

Table 3 XPS binding energy values for Ce0.9Ni0.1OY sample Elements

Ce0.9Ni0.1OY-cc

Ce0.9Ni0.1OY-cc-used

Refs.

Ni 2p

855.4, 861.7

855.4, 861.6

Ce 3d O 1s

882.5, 916.7 529.7, 532.1

882.5, 916.7 530.7, 533.0

Ce4+/Cetot

9.3%

10.5%

854.2 free NiO [32]; 852.3, 868.0 Ni0; 855.4, 861.7, 873.6 NiO2+; 858.1 NiT2+ [33] 882.6 Ce4+, 888.2 Ce3+ [35] 531.7 OH
/CO32
[37], 529.3 Ce4+, 533.8 Ce4
d [36] 13.7% for pure CeO2

W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33

Fig. 7. The stability of Ce0.8Ni0.2OY for partial oxidation of methane at 650 8C.

3.5. The stability study of Ce0.8Ni0.2OY catalyst The stability of the catalysts was performed on Ce0.8Ni0.2OY at 650 8C with a stoichiometric CH4/O2 feed ratio of 2 at a resident time of 3 s, as shown in Fig. 7. This temperature was selected since it was below the calcination temperature, while allowing H2/CO ratios close to 2. The residence time in the reactor was maintained at 3 s, with unchanged CH4/O2 feeding ratio. In spite of observable change in the composition of the outlet gas, the catalyst exhibited a very stable activity.

Fig. 8. The TPR profiles of Ce0.8Ni0.2OY after different treatment. (a) Fresh sample, (b) after reaction from 500–900–650 8C, (c) after reaction from 500 to 900 8C.

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Conversion of methane and oxygen increased regularly along the run, the final values being approx. 50% higher than their initial levels (Fig. 7). In addition, CO selectivity increased during the 10 first hours from 70 to 80% while the selectivity of H2 decreased rapidly to 70%, and the two selectivity values attained afterwards their steady levels. In the transient period, H2/CO ratio decreased from 2.4 to 2.1, and stabilized around 2.1. The carbon balance was higher than 90%, and no visible deactivation of the catalyst was observed throughout the 160 h run. The TPR experiments were carried out over the used Ce0.8Ni0.2OY sample to understand its role in the reaction. Comparison of the two used catalysts could be clearly established by TPR investigations, as showed in Fig. 8. The profile of the catalyst used in the 500–900–650 8C cycle largely differs from that of sample stopped at 900 8C, with higher H2 consumption at low temperature, and higher temperature of the peak maximum. Furthermore, the consumed H2 amount of the sample after 500–900–650 8C cycle is very close to the one consumed by the fresh sample, indicating that the reduced NiO in the reaction at 900 8C can be re-oxidized when reaction was performed at 650 8C for the second time. The stability of the catalyst at this temperature was attributed to the occurrence of the redox cycle of the nickel in the reaction. 4. Discussion The characterization results showed the strong interaction happened between Ce and Ni. Only metal Ni peaks are found in the XRD patterns after reaction. XPS results showed that most of Ni species on the surface is Ni2+ for both the fresh and used samples. TPR results also suggested that there is part of Ni2+ in the used sample. Therefore, it can be concluded that Ni2+ and Ni0 coexisted during the reaction. Combined with reaction results, although gaseous oxygen presented (no 100% O2 conversion was found), the metal Ni still exists even in the reaction at 900 8C, indicating that the reduced Ni0 should be oxidized to Ni2+ by latticed oxygen transferred from CeO2, instead of gaseous oxygen. The low Ni content samples show strong interaction between Ce and Ni, so the elector transferred easily. Consequently, the highest unreduced Ni2+/Nitot was found in the lowest Ni content sample listed in Table 2. In Ce-Ni-O catalysts, the reduction peak of CeO2 shifts further to lower temperature due to the H2 spillover effect, indicating that the CeO2 is much easier to be reduced in the present of Ni. The easier reducibility of ceria makes it possible to transfer oxygen species much more effectively via a redox cycle [38]. The strong ability to store, release and transfer oxygen species results in an enhanced ability to clean carbon that would normally accumulate on the Ni surface during the decomposition of CH4, which is the reason of the stable performance observed in the reaction. For CeO2, a reduction extent of about 30–35% was found for fresh samples, and H2 consumption of lattice oxygen in the used samples is higher than corresponding fresh samples. The reduction extent of lattice CeO2 decreased with Ni content for fresh samples, and it increased with Ni content after reaction, as shown in Fig. 6,

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which indicated that the Ce3+ is higher in fresh samples with higher Ni content. XPS showed that Ce3+ present for Ce0.9Ni0.1OY and Ce4+ content increase after reaction, and the surface O/Ce ratio increased about 50% after reaction. It is strongly suggested that the Ce3+ with oxygen vacancy in fresh sample take part in the reaction, and part of Ce3+ was oxidized to Ce4+ due to the oxygen vacancy was replenished by oxygen during the reaction. For the mechanism of partial oxidation of methane on Ni catalysts, it is generally accepted that the active component in supported nickel catalysts for the reaction is metallic nickel [39]. Hydrogen produced by methane decomposition on the catalyst can reduce surface nickel to start the partial oxidation of methane reaction. The reaction does not proceed without a reduced state of nickel on the catalyst because methane decomposition does not occur [40]. For the Ce-Ni-O catalysts, it was suggested [26] that the CH4 adsorbs on the metallic Ni and then dissociates further to give H2 and carbon species (CHX, X = 0–3), which can easily migrate to the interface of NiCeO2 due to the large extent of the boundary between CeO2 and Ni, and reduce CeO2 support near the metallic nickel particle to produce CO or, in the absence of reducible lattice oxygen, form carbon deposition on the metallic Ni or migrate to the surface of CeO2 support. The active oxygen originating from lattice oxygen was suggested by some authors. Otsuka et al. has reported that the gas–solid reaction between methane and cerium oxide (CeO2) directly produced a synthesis gas with H2/CO ratio of 2. They strongly suggested that the bulk lattice oxygen of CeO2 must participate in the oxidation of CH4 because the degree of reduction of CeO2 was 21% after reaction, comparing to the value (25%) by assuming that CeO2 is completely reduced to Ce2O3 [41]. Lefferts et al. studied the partial oxidation of methane to syngas on yttrium-stabilized zirconia (YSZ) [42– 44]. They found that despite the presence of adsorbed oxygen species, as confirmed by isotopic exchange experiments under reaction conditions, methane is selectively oxidized by lattice oxygen ions on the surfaces of YSZ and ZrO2. Choudhary et al. has reported that the autothermal reforming of methane to syngas over NiCoMgCeOX catalysts, it was found that the presence of cerium oxide in the catalyst not only provides an oxygen storage capacity, but also greatly enhances the mobility of lattice oxygen in the catalyst [28]. In the present study, it was found that Ce3+/Ce4+ and Ni2+/ 0 Ni existed in the reaction. It was suggested that there are two active sites, i.e., Ni0 and Ce3+ with oxygen vacancy. And the two active sites transfer during the reaction. The redox cycle happened between Ce3+/Ce4+ and Ni2+/Ni0 by 2+ 3+ 0 4+ Ni + Ce + & ! Ni + Ce , which provided Ce3+ and oxygen vacancy needed for CH4 active. And once Ce3+ was oxidized to Ce4+, Ni0 produced at the same time, it also is active site for the conversion of CH4 to synthesis gas. The active sites transfer between Ce3+/& and Ni0 in the reaction depending on the available active oxygen. But the CH4 conversion rate is not possible same on Ce3+/& and Ni0. The high mobility of lattice oxygen also contributed to the removal of carbon species on the surface, which resulted the

formation of CO or CO2. When the CH4 active rate and carbon species elimination rate by active lattice oxygen reach a balance, the stable reaction behavior was available, otherwise, the carbon deposition happened, resulting in the catalyst deactivation. 5. Conclusion Ce1
XNiXOY (X = 0.05–0.6) catalysts prepared by complexation–combustion method showed good catalytic activity and stability for the synthesis gas production from partial oxidation of methane. Increasing the Ni content in the mixed catalysts improved noticeably the selectivity of both H2 and CO, with enhanced methane conversion. The various characterisation techniques were used for the catalysts prepared before and after use in the temperature range 500–900 8C. It was found that the Ce3+/Ce4+ and Ni0/Ni2+ couples were involved in the reaction, with occurrence of the reaction Ni2+ + Ce3+ ! Ni0 + Ce4+. The present of Ni improved the mobility of CeO2 lattice oxygen. Both Ni0 and Ce3+ with oxygen vacancy are the active sites for the reaction. The gaseous oxygen was activated by the oxygen vacancy, resulted in the active lattice oxygen in the catalyst, which is active for the removal of carbon species originated from CH4 dissociation to produce CO and CO2. This result is highly promising because of the high stability of the developed catalysts, in particular in a temperature range allowing H2/CO ratio close to 2, corresponding to syngas production. Acknowledgments This work was fund by FFCSA postdoctoral fellowship, Region Lorraine and INPL, together with China Scholarship Council (CSC). The authors would like thank Yvan Zimmermann and Suzanne Libs for their assistance with some characterization works. References [1] G.E. Keller, M.M. Bhasin, J. Catal. 73 (1982) 9. [2] J.S. Lee, S.T. Oyama, Catal. Rev. Sci. Eng. 30 (1988) 249. [3] Y. Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka, A.R. Sanger, Catal. Rev. Sci. Eng. 33 (1990) 163. [4] L. Guczi, R.A. van Santen, K.V. Sarma, Catal. Rev. Sci. Eng. 38 (1996) 249. [5] R. Pitchai, K. Klier, Catal. Rev. Sci. Eng. 28 (1986) 13. [6] N.R. Foster, Appl. Catal. A: Gen. 19 (1985) 1. [7] T.J. Hall, J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner, S.H. Taylor, Fuel Process. Technol. 42 (1995) 151. [8] M.C. Iliuta, B.P.A. Grandjean, F. Larachi, Ind. Eng. Chem. Res. 42 (2) (2003) 323. [9] L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett. 21 (1–2) (1993) 35. [10] P.L. Tan, Y.L. Leung, S.Y. Lai, C.T. Au, Catal. Lett. 78 (1–4) (2002) 251. [11] A.P.E. York, T. Xiao, M.L.H. Green, Top. Catal. 22 (3–4) (2003) 345. [12] M. Prette, C. Eichner, M. Perrin, Trans. Faraday Soc. 43 (1946) 335. [13] Y.H. Hu, E. Ruchenstein, Adv. Catal. 48 (2004) 297. [14] V.A. Tsipouriari, Z. Zhang, X.E. Verykios, J. Catal. 179 (1998) 283. [15] S. Tang, J. Lin, K.L. Tan, Catal. Lett. 51 (1998) 169. [16] K. Takehira, T. Shishido, M. Kondo, J. Catal. 207 (2002) 307.

W. Shan et al. / Applied Catalysis A: General 311 (2006) 24–33 [17] T.L. Zhu, M. Flytzani-Stephanopoulos, Appl. Catal. A: Gen. 208 (2001) 403. [18] L. Cao, Y. Chen, W. Li, Stud. Surf. Sci. Catal. 107 (1997) 467. [19] S.L. Liu, G.X. Xiong, S.S. Sheng, Q. Miao, W.S. Yang, Stud. Surf. Sci. Catal. 119 (1998) 747. [20] R. Shiozaki, A.G. Andersen, T. Hayakawa, S. Hamakawa, K. Suzuki, M. Shimizu, K. Takehira, Stud. Surf. Sci. Catal. 110 (1997) 701. [21] Y.H. Zhang, G.X. Xiong, S.S. Sheng, W.S. Yang, Catal. Today 63 (2000) 517. [22] T. Hayakama, H. Harihara, A.G. Andersen, K. Suzuki, H. Yasuda, T. Tsunoda, S. Hamakawa, A.P.E. York, Y.S. Yoon, M. Shimizu, K. Takehira, Appl. Catal. A: Gen. 149 (1997) 391. [23] E. Ruckenstein, Y.H. Hu, Appl. Catal. A: Gen. 133 (1995) 149. [24] F. van Looij, J.W. Geus, J. Catal. 168 (1997) 154. [25] Y. Lu, Y. Liu, S. Shen, J. Catal. 177 (1998) 386. [26] W.S. Dong, K.W. Jun, H.S. Roh, Z.W. Liu, S.E. Park, Catal. Lett. 78 (1–4) (2002) 215. [27] W. Shan, W. Shen, C. Li, Chem. Mater. 15 (2003) 4761. [28] V.R. Choudhary, K.C. Mondal, A.S. Mamman, J. Catal. 233 (2005) 36. [29] W. Shan, M. Luo, P. Ying, W. Shen, C. Li, Appl. Catal. A: Gen. 246 (2003) 1. [30] G. Wrobel, M.P. Sohier, A. D’Huysser, J.P. Bonnelle, Appl. Catal. A: Gen. 101 (1993) 73.

33

[31] H.S. Roh, K.W. Jun, W.S. Dong, S.E. Park, Y.S. Baek, Catal. Lett. 74 (2001) 31. [32] A.A. Lemonidou, M.A. Goula, I.A. Vasalos, Catal. Today 46 (1998) 175. [33] G. Poncelet, M.A. Centeno, R. Molina, Appl. Catal. A: Gen. 288 (2005) 232. [34] J.Z. Shyu, W.H. Weber, H.S. Gandhi, J. Phys. Chem. 92 (1988) 4964. [35] B. Ska˚rman, D. Grandjean, R.E. Benfield, A. Hinz, A. Andersson, L.R. Wallenberg, J. Catal. 211 (2002) 119. [36] M.S.P. Francisco, V.R. Mastelaro, P.A.P. Nascente, A.O. Florentino, J. Phys. Chem. B 105 (2001) 10515. [37] J. Requies, M.A. Cabrero, V.L. Barrio, M.B. Gu¨emez, J.F. Cambra, P.L. Arias, F.J. Pe´rez-Alonso, M. Ojeda, M.A. Pen˜a, J.L.G. Fierro, Appl. Catal. A: Gen. 289 (2005) 214. [38] J.S. Chang, S.E. Park, H. Chon, Appl. Catal. A: Gen. 145 (1996) 111. [39] D. Dissanayake, M.P. Rosynek, K.C.C. Kharas, J.H. Lunsford, J. Catal. 132 (1991) 117. [40] M. Ito, T. Tagawa, S. Goto, J. Chem. Eng. Jpn. 32 (3) (1999) 274. [41] K. Otsuka, Y. Wang, E. Sunada, I. Yamanaka, J. Catal. 175 (1998) 152. [42] J. Zhu, J.G. van Ommen, H.J.M. Bouwmeester, L. Lefferts, J. Catal. 233 (2005) 434. [43] J. Zhu, J.G. van Ommen, L. Lefferts, J. Catal. 225 (2004) 388. [44] J. Zhu, J.G. van Ommen, A. Knoester, L. Lefferts, J. Catal. 230 (2005) 391.